Flat-panel displays are being developed which utilize liquid crystals or electroluminescent materials to produce high quality images. These displays are expected to supplant cathode ray tube (CRT) technology and provide a more highly defined television picture. The most promising route to large scale high quality liquid crystal displays (LCDs), for example, is the active-matrix approach in which thin-film transistors (TFTS) are co-located with LCD pixels. The primary advantage of the active matrix approach using TFTs is the elimination of cross-talk between pixels, and the excellent grey scale that can be attained with TFT-compatible LCDs.
Flat panel displays employing LCDs generally include five different layers: a white light source, a first polarizing filter that is mounted on one side of a circuit panel on which the TFTs are arrayed to form pixels, a filter plate containing at least three primary colors arranged into pixels, and finally a second polarizing filter. A volume between the circuit panel and the filter plate is filled with a liquid crystal material. This material will rotate the polarization of light when an electric field is applied across it between the circuit panel and a ground affixed to the filter plate. Thus, when a particular pixel of the display is turned on, the liquid crystal material rotates polarized light being transmitted through the material so that it will pass through the second polarizing filter.
Preferred embodiments of the present invention relates to projection display devices (i.e. monitors and image projectors) including methods of fabricating such devices using thin films of single crystal silicon in which a light valve matrix (or matrices) is formed for controlling images produced by these devices. In accordance with the present invention, projection display devices employing high density single crystal silicon light valve matrices provide high resolution images compatible with 35 mm optics.
In one preferred embodiment, an optically transmissive substrate is positioned to receive light from a back-light source and a light valve matrix is secured to the substrate. In accordance with the present invention, the light valve matrix includes an array of transistors and an array of electrodes which are formed in the thin film of single crystal silicon. The light valve matrix also includes an adjacent light transmitting material, through which light from the back-light source is selectively transmitted. Preferred embodiments are directed to light valves employing a transmissive light transmitting material such as liquid crystal or a ferroelectric material, although other transmissive materials may be used. Each light valve includes a transistor, an electrode and a portion of the adjacent light transmitting material. Each transistor, by application of an electric field or signal, serves to control the optical transmission of light through the adjacent light transmitting material for a single light valve.
A driver circuit is electrically connected to the light valve matrix to selectively actuate the light valves. The drive circuitry may be formed in the same thin-film material in which the transistors and electrodes have been formed. The drive circuitry is capable of being fully interconnected to the matrix using thin-film metallization techniques without the need for wires and wirebonding. An optical system is also provided for projecting light transmitted through the actuated light valves onto a large viewing surface.
The present devices and related methods for fabricating projectors satisfy the requirements of large screen television or monitor displays for producing highly defined color images. To that end, a projection display device can have multiple light valves each adapted to selectively transmit light of a single primary color. Further, a dichroic prism may be provided for combining the single color light transmitted by each light valve producing a multi-color light image which is projected onto a large viewing surface.
Other preferred embodiments of the present invention relate to an active matrix display panel adapted for use in a conventional 35 mm slide projector for providing monochrome or multi-color images. The display panel is fabricated to have equivalent physical dimensions as a standard 35 mm photographic slide having an image which can be projected by a slide projector. In accordance with the present invention, the active matrix display panel, being packaged to be size-equivalent with a standard 35 mm slide, is insertible into a slide projector with modification thereof for generating the projected images. An electronics unit is connected to the display panel and controls image generation by the active matrix. In preferred embodiments, the display panel is capable of generating monochrome or multi-color images.
In one preferred embodiment of the invention, an active matrix display device is adapted for use with a slide projector having a projector body, a light source, an optical system and a chamber in which a 35 mm slide can be placed for projection of its image onto an external viewing surface. The display device includes a housing and an active matrix display panel movably mounted to the housing. As such, the display panel has a storage position and an operating position. The housing is positioned on the slide projector body such that the display panel, being moved into the operating position, can be securely disposed in the projector chamber for selectively transmitting light from the light source to provide images for projection by the slide projector.
The housing preferably contains a shielded electronics assembly which is electrically connected to the display panel for controlling image generation. The electronics assembly receives image data from an image generation device which may be a computer or any video device. Image data provided by the device is processed by the electronics and sent to the active matrix display panel. Responsive to the received data, the individual active matrix light valves are activated such that the illuminating light from the light source is selectively transmitted through the active matrix to form monochrome or multi-color images.
In another preferred embodiment, the active matrix display device includes an active matrix display panel and a remote electronics housing. The display panel is dimensioned to be securely positioned in the chamber of the slide projector and is electrically connected to electronics in the remote housing by a cable.
In yet another preferred embodiment, the active matrix display device includes an active matrix display panel which is not physically connected to the electronics housing. Instead, the active matrix display panel and the electronics in the housing communicate with each other via antenna elements such as RF antennas or infrared transmitter/detector elements.
As with aforementioned embodiments, an active matrix display panel has an array of pixels or light valves which are individually actuated by a drive circuit. The drive circuit components can be positioned adjacent to the array and electrically connected to the light valves. As such, the individual light valves are actuated by the drive circuit so that illuminating light is selectively transmitted through the active matrix to form an image.
In preferred embodiments, the active matrix circuitry is formed in or on a layer of a semiconductor material such as silicon. It is noted that any number of fabrication techniques can be employed to provide preferred thin-films of polysilicon or single crystal silicon. In embodiments in which a thin-film of single crystal silicon is used, extremely high light valve densities can be achieved such that high resolution images are obtained. Other embodiments employ the use of a solid state material or any material whose optical transmission properties can be altered by the application of an electric field to supply the light valves of the present invention.
A preferred embodiment of the fabrication process for a liquid crystal transmission display comprises providing a thin film of an single crystal semiconductor material such as silicon. In one embodiment, the processing steps for forming a thin film of single crystal silicon include forming a layer of polysilicon over a supporting substrate and scanning the layer with a heat source to melt and recrystallize the polysilicon to form a thin film of essentially single crystal silicon. In another embodiment, a single crystal silicon film or layer can be formed by a SIMOX (Separation by IMplantation of OXygen) process. In another embodiment, the wafer of single crystal silicon can be secured on a quartz substrate utilizing Van der Waals bonding and the wafer can be thinned using known techniques to provide the thin film semiconductor. In yet another embodiment, a bonded wafer approach can be used to form the layer of thin film single crystal silicon on a single crystal silicon wafer.
The process also comprises the step of forming an array of transistors or switching circuits, an array of pixel electrodes and drive circuitry in or on a front side of the thin film single crystal silicon such that each pixel electrode is electrically connected to one of the switching circuits to provide an active matrix array of pixel elements. Each pixel element is actuatable by one of the switching circuits, and the drive circuitry is used to control pixel actuation.
In accordance with the present invention, the process includes the step of forming an array of color filter elements over the front side of the thin film of essentially single crystal silicon material. Each color filter element is correlated with one (or more) of the pixel elements. The color filter elements are formed by applying a carrier layer such as an emulsion or a photoresist, including the appropriate dye, on or over the pixel elements, and then processing the carrier layer to provide an array of filter elements. Alternatively, the color filter elements can be formed by direct deposition of a conventional filter material such as single layer or multiple layers of thin film optical coatings. In either case, the layer is then processed and patterned to produce a resulting color filter element adjacent to each of a plurality of pixel elements for one color. This process can be repeated to provide different color filter elements for the remaining pixel elements to produce a multicolor display. A matrix array of opaque (or black) elements can also be formed on or over portions of the thin film of single crystal silicon such that the opaque elements are interspersed with the color filter elements. Each opaque element can be used to define the perimeter of each pixel element and serves to absorb incident light that would otherwise imping upon the switching circuit associated with the pixel element. Preferably, a layer of aluminum or the like is also formed over one or both side of the thin film and patterned such that each aluminun element serves as a light shield to reflect light that may otherwise be directed at the switching circuits or interconnects to the drive circuitry.
The display fabrication process also includes the step of transferring the thin silicon film, upon which the active matrix has been formed, and adjacent color filter array from the supporting substrate onto an optically transmissive substrate. This will expose a planar surface which in one embodiment can correspond to an insulating layer adjacent to the back side of the film or alternatively it will correspond to the back side of the film if the insulating layer is removed. The transfer step includes forming an optically transmissive isolation (barrier) layer, which can comprise polyimide, nitride, oxide or sputtered glass, over the color filter array. The thin film is then attached to the optically transmissive substrate with an adhesive such that the isolation layer serves to isolate the filter elements from each other and the adhesive. A light transmitting liquid crystal material is then formed adjacent to the planar surface associated with the silicon thin film and a counterelectrode is formed adjacent to the liquid crystal material. The counterelectrode is associated with the array of pixel elements such that an electric field generated by each pixel element alters a light transmitting property of the light transmitting material.
Other preferred embodiments of the present invention are directed to emissive color displays employing a color filter for displaying color images and methods of fabricating such displays. In one preferred embodiment, an electroluminescent (EL) color display includes an active matrix circuit panel formed over a supporting substrate. As described above, the circuit panel comprises a thin film (about 0.1-2.0 microns) of single crystal or essentially single crystal semiconductor material. An array of transistors or switching circuits, an array of pixel electrodes and a driver circuit are formed in or on the thin film. An electroluminescent material is positioned adjacent to the circuit panel circuitry and patterned to form an array of EL elements.
For the EL display, each transistor, the associated pixel electrode and the associated EL material element are referred to as a pixel element or light emitter. For each pixel element, the pixel electrode is electrically connected to one of the transistors which is capable of generating an electric field or signal across the adjacent EL material causing the emission of light by the EL material. The driver circuit can be formed in or on the same single crystal material as the active matrix circuitry. The driver circuit is capable of being fully interconnected to the transistors for actuating the pixel elements using thin film metallization techniques without the need for wires and wirebonding.
An optically transmissive electrode is positioned over the EL structure which can comprise a white phosphor. As such, the electric field generated at each pixel element lies between the optically transmissive electrode and the pixel electrode. An array of color filter elements is formed adjacent to a surface of the electrode. Each color filter element is correlated with one pixel element. The color filter elements are formed by processing, in accordance with the techniques described herein, an emulsion, a photoresist or other suitable carrier in which a dye is positioned or other conventional filter materials. The presence of the field causes the EL material to generate light which passes through the color filter element to produce a colored light. As such, each pixel element of the EL display can be an independently controlled color light emitter whose light emitting properties are altered by the electric field or signal. The present invention comprises methods for fabricating EL displays capable of producing high definition color images. A preferred embodiment of the EL display fabrication process comprises providing a thin film of an essentially single crystal semiconductor material such as silicon. The processing steps for forming a thin film of essentially single crystal silicon include forming a layer of polysilicon over a supporting substrate and scanning the layer with a heat source to melt and recrystallize the polysilicon to form a thin film of essentially single crystal silicon. Alternatively, the single crystal silicon film or layer can be formed by a SIMOX process, Van der Waals bonding of a wafer to quartz or a bonded wafer approach as described in greater detail below.
The process also comprises forming an array of transistors, an array of pixel electrodes and drive circuitry in or on the thin film of single crystal silicon such that each pixel electrode is electrically connected to one of the transistors to provide an active matrix array of pixel elements or light emitters. Each pixel element is actuatable by one of the transistors, and the drive circuitry is used to control pixel actuation. The process also includes forming a layer of EL material (such as a white phosphor) adjacent to the circuit panel circuitry and patterning the material to form an array of EL elements. An optically transmissive electrode is then formed adjacent to the EL structure. An array of color filter elements are then formed over the electrode. Each color filter element is correlated with one (or more) of the pixel elements.
The color filter elements are formed by applying a carrier layer such as an emulsion or a photoresist to the thin film. The carrier layer is then processed and patterned to produce a resulting color filter element adjacent to each of a plurality of pixel elements. This process can be repeated to provide different color filter elements for the remaining pixel elements to produce an emissive active matrix color display. A pattern of opaque (or black) elements can also be formed such that the opaque elements are interspersed with the color filter elements. The EL display structure is completed by forming an optically transmissive layer over the color filter array.
The EL display fabrication process can also include the step of transferring the structure from the supporting substrate onto an optically transmissive substrate such as glass, plastic or a head-mounted visor. The transfer steps can include attaching the display structure to a temporary substrate, removing the supporting substrate, attaching the optically transmissive substrate and removing the temporary substrate.
A critical advantage provided by the above referenced methods of color filter fabrication for display panels is that they provide for precise alignment of the pixel elements with the filter elements. Whereas conventional color filter systems involve alignment of filter elements on the opposite side of the liquid crystal material, for example with the pixel elements in the active matrix when the laminated structure of the display is finally assembled, the present system provides for alignment by fabricating the filter elements directly on the circuit panel. This provides particular advantages when utilizing transfer methods as the processing involved in the transfer can result in some shrinkage of portions or all of the display thereby making precise alignment with conventional filter arrays more difficult.